Calcium phosphate, especially hydroxyapatite, has excellent affinity to the living body and an excellent adsorptivity and hence has been studied as to the utilities thereof in a variety of fields, particularly use thereof as implant material for replacing or repairing hard tissues of living bodies. The implant material is required to have biodynamic strength in addition to an affinity to living body. Nevertheless, hydroxyapatite is not satisfactory in terms of the strength even in the form of a sintered product. Accordingly, from a practical viewpoint, it is favorable to use a substrate or core material such as metallic materials, ceramics, glass, etc. and to form a hydroxyapatite coating film on the surface of said substrate or core material.
Various methods have hitherto been proposed for forming a coating film of calcium phosphate. For example, a thermal plasma spray method (cf. Japanese Patent First Publication (Kokai) No. 82893/1977), a spattering method (cf. Japanese Patent First Publication (Kokai) No. 109049/1983), a physical vapor deposition (PVD) or chemical vapor deposition (CVD) method (cf. Japanese Patent First Publication (Kokai) No. 111753/1984), an electrophoretic method (cf. Japanese Patent First Publication (Kokai) No. 128190/1978), and a coating method (cf. Japanese Patent First Publication (Kokai) No. 118411/1978) has been proposed.
However, the thermal plasma spray method, spattering method, CVD method and PVD method are with difficulty applied to a substrate having a complicated shape, for example, onto the inner surface of porous substrate electrophoretic method can not be used to form the coating film onto a substrate having no electrical conductivity. On the other hand, the coating method is advantageously easy in the treatment, and the above-mentioned Japanese Patent First Publication (Kokai) No. 118411/1978 discloses a method for forming a coating film by suspending fine particles of apatite in water and coating the aqueous suspension onto the surface of a substrate, followed by calcining the coated substrate. However, this coating method still exhibits a problem in that it is usually difficult to prepare very fine particles of apatite and the particles are easily agglomerated, and further, the particles dispersed in water have usually a particle size of more than 0.5 .mu.m, and hence, the apatite has less adhesiveness with respect to the substrate surface and the coating film is easily peeled off from the substrate. The coating method may also be carried out by coating a suspension of fine particles of apatite in the presence of a water soluble high molecular substance on a surface of a substrate, followed by calcining the coated substrate, thereby burning off the high molecular substance. According to this method, the adhesiveness is improved, but it can not be applied to a material such as titanium which is denatured at a calcining temperature.